It is known that linear or slightly branched olefins of relatively low molecular weight may be reacted in the gaseous phase with water vapour at elevated pressures and temperatures to form alcohols. Of particular industrial significance is the synthesis of ethanol from ethene and isopropanol from propene. Production of these alcohols proceeds in the presence of acidic catalysts, wherein as a rule a phosphoric acid-impregnated, aluminosilicate or silicate material is used as the catalyst support.
The material of the catalyst support either comprises pure silicic acid, such as for example silica gel (U.S. Pat. No. 2,579,601), or consists of silicic acid with varying alumina contents (U.S. Pat. No. 3,311,568) or of pure, for example montmorillonite-containing phyllosilicates (DE 29 08 491).
Apart from these phosphoric acid-containing catalyst supports, zeolite materials (EP 0 323 269 B1) or other acidic catalysts, such as zirconium phosphates (GB 005 534) for example, are also used.
In the case of supports based exclusively on silicic acid in the form of silica gels, mechanical strength over a relatively long service life has so far proved problematic. Aluminium-containing catalyst supports or those based solely on alumina exhibit markedly greater long-term stability but have the considerable disadvantage that aluminium leaches out of the catalyst support through the action of the phosphoric acid during the hydration reaction. The aluminium reappears as a sparingly soluble deposit in the form of aluminium phosphates in the downstream apparatus. These become gradually clogged thereby.
DE 1 156 772 discloses a process for reducing the aluminium content of the phyllosilicates through the action of hydrochloric acid. However, the support material still comprises a residual aluminium content of approximately 1 to 2 wt. % even when washed intensively with hydrochloric acid.
EP 0 578 441 B1 achieves a degree of long-term stability by using a pelletised Aerosil-based silicate support (Degussa) containing no aluminium. The starting material for Aerosil production is silicon tetrachloride, which is relatively expensive. Since phyllosilicate-based materials, such as montmorillonite for example, are natural products which may be excavated from appropriate deposits, they have a clear advantage over pelletised silicate supports from the point of view of the economic viability of the hydration process.